1. Field of the Invention
The present invention relates to a pressure sensor.
2. Description of Related Art
Patent Document 1 recites a pressure sensor. According to Patent Document 1, a pressure sensor is disposed in an internal space of a door of a vehicle in order to sense a collision taking place at a side of the vehicle. When an object collides with the door, the pressure sensor senses a change in pressure of the internal space of the door. For detection of such pressure change, the pressure sensor includes an introduction passage. The introduction passage introduces the air in the internal space of the door into a pressure detection surface of the pressure sensor. The introduction passage is formed to extend in a direction perpendicular to the pressure detection surface of the pressure sensor, that is, in a horizontal direction.
Patent document 2 recites another pressure sensor. According to Patent Document 2, a pressure sensor has an introduction passage, which leads to a pressure detection surface of the pressure sensor in a similar manner to that according to Patent Document 1. The introduction passage is formed to extend in a direction perpendicular to the pressure detection surface of the pressure sensor.
Patent document 3 recites another pressure sensor. A pressure sensor includes a sensing element covered or coated by a gelled resin. When an air pressure applied to the sensing element varies, the gelled resin is deformed. Pressure is transmitted to a sensor circuit accordingly. A change in pressure is detected using a sensor circuit.    Patent Document 1: IP-A-2006-306155 corresponding to US-A-2006/0237255    Patent Document 2: IP-A-2003-4570    Patent Document 3: IP-A-2001-116639
The internal space of a door of a vehicle typically gives an environment where an object may get wet. When a pressure sensor is disposed in such an environment, it is necessary to restrict water accumulation in the sensor.
When the sensing element disclosed in Patent Document 3 is used as a sensing element of a pressure sensor, it may be possible to prevent a sensor circuit from directly getting wet. However, according to a pressure sensor disclosed in Patent Document 1, since an introduction passage is formed to extend in a horizontal direction, water can enter the introduction passage.
When such a pressure sensor is used in a remarkably low-temperature environment, water in the introduction passage may freeze on a surface of a gelled resin or a pressure detection surface of the sensing element. When an ice sticks to the pressure detection surface of a sensing element, a change in pressure of the internal space of a door causes less deformation of the gelled resin. It becomes thus difficult to detect a pressure with high accuracy. In addition, when the body (i.e., the door) in which the pressure sensor is disposed is displaced, an inertia force acts on the ice that sticks to the pressure detection surface. Due to the inertia force acting on the ice, the sensing element may sense a pseudo pressure change. To avoid the above-described difficulties, it may be necessary to configure a pressure sensor so that water does not stay on the pressure detection surface.